The invention relates to optical object detection.
Typical optical object detectors illuminate an area with light, usually in the infra-red region of the light spectrum. Any object entering the illuminated area will reflect some of the light. A photodetector circuit detects the reflected light. If the object is larger than a minimum size, the detected signal exceeds a threshold and causes an output signal to be generated indicating the object""s presence. To detect as small an object as possible at the greatest distance possible, the photodetector circuit must be as sensitive as possible. There are three main ways to increase the sensitivity of the photodetector circuit:
1. Increase the signal to noise ratio of the signal resulting from the intended light source being reflected by the object.
2. Reduce the effect of interfering signals resulting from non-intended light sources directly entering the photodetector.
3. Reduce the effects of drift mechanisms, such as temperature fluctuations, on the detection threshold.
Non-intended light sources form a varying ambient signal in the photodetector circuit. The desired signal (due to the intended illuminating light source, reflected by the object) rides on top of this ambient signal. As the ambient signal increases, it causes more current to flow in the photodetector. The increased current results in increased detector noise. This is called detector noise floor modulation. For maximum sensitivity, the threshold that the desired signal must overcome is set as low as possible. Both the ambient signal and the increased noise resulting from it force the threshold to be increased, thus reducing sensitivity. Also, as the ambient signal increases, it causes the desired signal to decrease due to non-linear effects in the photodetector. This is called detector saturation. This reduces the signal to noise ratio of the desired signal, again reducing sensitivity.
Many techniques have been used to improve sensitivity. One such technique is to chop the intended illuminating light source at a fixed frequency (called the carrier frequency) and filter the intended signal from the photodetector to pass only this frequency. This technique rejects all ambient signals outside the bandwidth of the carrier filter. It also increases the signal to noise ratio of the desired signal by rejecting the noise outside the bandwidth of the carrier filter. An extension of this technique is to use synchronous detection. Synchronous detection rejects ambient signals and noise that are not in phase with the carrier frequency, by using a product detector instead of a diode detector. Synchronous detectors require the carrier frequency signal to be routed into the detector circuitry for the purpose of product detection. The carrier frequency signal must be very well isolated from the optical signal amplifier otherwise the routed signal will swamp the optical detected signal. Such high isolation increases product cost. This carrier frequency filtering technique and its synchronous detection extension do not reduce the effects of detector noise floor modulation or detector saturation caused by ambient signals within the bandwidth of the carrier filter. Since higher frequency ambient signals have lower amplitudes, a higher carrier frequency will reduce the effects of ambient signals. But since photodetector preamp noise increases with frequency, a higher carrier frequency decreases the signal to noise ratio of the desired signal. Thus, there is an optimum carrier frequency.
Another technique is to incorporate an automatic level control (ALC) in the photodetector circuit. The ALC maintains the peak of the detected signal at a fixed amplitude by varying the gain of an amplifier. This technique reduces the effects of ambient signal, including detector noise floor modulation, but only if the ambient signal varies at a rate slower than the response time of the ALC. The response time of the ALC must be slower than the carrier period (carrier period is the inverse of the carrier frequency), otherwise the desired signal amplitude will be reduced. The ALC also prevents the photodetector amplifier from being saturated by too strong a signal (saturation is a common problem due to the high gain required for sensitive detection). The ALC can also help reduce the effects of drift mechanisms on the detection threshold.
Other techniques which prevent amplifier saturation due to strong ambient signals are electrical ambient cancellation circuitry and optical filtering. The former cancels ambient signals at signal frequencies below the carrier frequency at the input of the preamp rather than at the output (this is done by negative feedback). The latter (optical filtering) is a window or an encasement around the photodetector which filters out all optical energy outside a bandwidth centered around the infra-red radiation frequency of the intended light source.
Finally, temperature compensation techniques reduce the effects of temperature on the detection threshold, allowing the threshold to be set closer to the noise floor.
My optical object detector uses a low gain linear photodetector preamplifier. The gain of the preamplifier is low enough to prevent saturation of the preamplifier output by strong ambient signals, but high enough to maintain good signal to noise ratio. The preamplifier output signal is filtered by a carrier band pass filter to filter out ambient signals and noise which are outside the bandwidth of the carrier filter. The output signal of the carrier filter is detected by a sensitive multistage logarithmic detector instead of a diode junction. Such a detector is typically the signal strength output of an intermediate frequency amplifier/demodulator integrated circuit (the normal demodulator output of the integrated circuit is not used).
The illuminating LED is chopped at three different frequency rates. The high frequency chopping rate is the carrier frequency. The medium frequency chopping rate is used to synchronously detect the illuminating signal reflected from an object and detected by the photodetector circuit. The low frequency chopping rate is used to give the photodetector circuit a noise reference to compare to (when the LED is not emitting light), so that the photodetector circuit can produce an output signal which is indicative of the signal to noise ratio of the signal resulting from the reflected light rather than just the amplitude of the signal compared to a fixed voltage level. The carrier frequency could also be used for synchronous detection, but this would be more expensive due to the high isolation required between the carrier signal and the preamp input circuit.